Echoes in the brain open a window on yesterday

Toss a stone into a pool and it leaves ripples long after it sinks. Ideas and experiences have a similar affect on our brains&colon; short bouts of intense neural activity leave ripples in the brain’s background activity that can still be detected 24 hours later. The finding effectively opens a window into a person’s recent past.

Even when you are doing nothing, the brain is busy. Cut off from external stimuli and left to “idle”, the brain enters a resting state. “You would expect it to quieten down,” says Rafael Malach at the Weizmann Institute of Science in Rehovot, Israel. But instead, the brain just switches gear, producing patterns of activity that are slower but no less noisy. “The activity is very organised, very rich and very consistent,” says Malach. But what it means is largely a mystery.

Yesterday today

Malach and his colleagues wondered whether the activity might in fact be a kind of echo. Could it tell us something about what the brain had been up to previously? “It might be a window into the previous day’s activity,” says Malach.

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To test the idea, the team compared fMRI scans of 20 people taken before, during and after a period of intense cognitive activity. They focused on a region of the brain called the dorsal anterior cingulate cortex, which is linked to decision-making and volition.

When in the scanner, participants were asked to concentrate their thoughts in a way that produced activity in this area. For example, they were asked to think about starting a new project.

To help the person in the scanner maximise their brain activity, feedback was provided&colon; the researchers played the participants a tone that went up or down in pitch as brain activity in the desired region increased or decreased. Each person was asked to try to keep the pitch as high as possible.

Six minutes after the concentration task, while the participants were still inside the scanner, the researchers took a snapshot scan of their brains in its resting state. They took a second snapshot scan 24 hours later, again while the participants’ brains were resting.

Echo builder

Using software to analyse the activity patterns, the team found that patterns of firing neurons observed during the concentration task could still be seen when the brain was in its resting state – though they had not been active before the task. Remarkably, the pattern was even stronger at the 24-hour mark.

The results are impressive, says Lila Davachi at New York University. She has previously shown a similar restructuring of brain activity after memory tasks, but this is the first time that it has been seen in a task not specifically involving memory.

“The way our brains are wired is a result of our experience,” Davachi says. And this is not only the case during development in children. It is easy to forget that the brain’s plasticity remains into adulthood, she says. “Every time we learn something new we’re changing our brain a little.”

Coarse signal

Sam Schwarzkopf at University College London also finds the results intriguing. “In theory, it seems possible that we could pick up more subtle things, like memories, feelings and associations,” he says.

But he stresses that brain-imaging methods for humans are still relatively blunt and only capable of picking up signals from large chunks of brain tissue that contain millions of neurons. “That anything can be read out from this suggests that the signal must be fairly coarse.”

Davachi agrees that our technology is not sophisticated enough to tease out individual thoughts, but says it is not totally out of the question that we could in future. “It’s possible that we might be able to get there.”

No spotless mind

Malach now hopes to see whether the findings hold across the cerebral cortex in general and whether the patterns will show up even in the brain’s non-resting states.

He thinks that one day we might be able to build a library of patterns linking particular experiences with the observable activity they produce. In theory, this could make it possible to read someone’s past from their resting brain waves – at least to a limited extent.

“We might have a method to tell us not just what we are thinking now, but what were the meaningful thoughts that we had in the last few days,” he says.

Reading off biases and tendencies, rather than thoughts, is another possibility. If someone is afraid of spiders, for example, then this phobia might also be observable in their resting brain activity, says Malach.